This invention relates to a pressure sensing device for sensing the pressure of a fluid and, in particular, a pressure sensing device using a diaphragm upon which pressure is exerted.
A pressure sensing device of this type is known as the so-called differential pressure transmitter or pressure transmitter. To explain it in more detail, for example, the differential pressure transmitter includes a first metallic diaphragm for defining a first chamber and a second metallic diaphragm for defining a second chamber. A predetermined fluid is sealed in the first and second chambers. The pressure of the first measuring fluid externally acts upon the first diaphragm, transmitting the pressure of the first measuring fluid to the internal fluid within the first chamber. The pressure of the second measuring fluid externally acts upon the second diaphragm, transmitting the pressure of the second measuring fluid to the internal fluid within the second chamber as in the case of the first chamber. A pressure-sensitive element is disposed between the first and second chambers. The pressure-sensitive element has a first surface responsive to the pressure within the first chamber and a second surface responsive to the pressure within the second chamber and is adapted to generate an electric signal corresponding to the differential pressure between the pressure in the first chamber and that in the second chamber. The above-mentioned differential pressure transmitter permits the pressures of the first and second measuring fluids to be transmitted to the first and second chambers, respectively, and detects the first and second measuring fluids through the use of the above-mentioned pressure-sensitive element.
In this connection it is to be noted that the pressure transmitter also permits the pressure within the internal chamber to be transmitted through the diaphragm and measures it on the pressure-sensitive element and that the pressure transmitter is operated basically on the same principle as that of the differential pressure transmitter.
As appreciated from the operation principle of the differential pressure transmitter and pressure transmitter, in order to obtain a highly exact output level on the pressure transmitter of this type, it is necessary to better transmit the pressure of the measuring fluid to the fluid within the internal chamber defined by the diaphragm, so that the pressure variation within the internal chamber can be followed, with high accuracy, with respect to the pressure variation of the measuring fluid. In order to satisfy such a requirement, either the thickness of the diaphragm should be decreased or the cushioning or springing characteristic of the diaphragm should be reduced with a greater effective diameter, so that the diaphragm may readily be deformed in accordance with the pressure of the measuring fluid.
A stainless steel, such as SUS316L (Japanese Industrial Standards), is usually used for the diaphragm for the known pressure transmitter. Where, however, the thickness of the diaphragm is reduced below a desired springing level so as to obtain an exact output characteristic, the mechanical strength of the diaphragm is reduced with the decreasing thickness of the diaphragm. As a result, the diaphragm is damaged, failing to reduce it in actual practice. For this reason, the stainless steel diaphragm has a drawback that the thickness of the diaphragm cannot be adequately reduced. For the stainless steel diaphragm, if the thickness is adequately reduced below the desired level, it is unavoidably necessary to employ a diaphragm of a greater effective diameter. As a result, the pressure transmitter per se becomes bulkier.